[rtate]

I recently got a Van Alstine model 3 amp that I just finished
"freshening up" I replaced all the electrolytics with black gates and most of the film and all of the ceramics caps with polyproplyene. Also I upped the main ps caps from 20,000@75v
to 32,000@80v.
The amp plays music fine but it has a hiss that sounds like white noise.
It starts out almost inaudible but after about 2 hours I can hear it from 10 feet away!!
I have a schematic for the amplifier boards but nothing for the power supply, speaker protection circuit or the "bootstrap" boards.
I don't know what the problem could be or where to start looking
Can someone point me in the right direction please??
This seems like a good amp and I would really like to fix it!!

[rlee8394]

Is the hiss on both channels, or just one?

Ron

[ArthurDent]

Have you given Frank a call ? If not you should, he's very generous in sharing his expertise. Busy as he is he'll probably offer his thoughts/suggestions here when he catches your thread.

Hope you find a speedy resolution. 

[rtate]

the hiss is on both channels....

[rlee8394]

Did this problem occur after you replaced the parts? If so, I would reverse the "freshening up" process starting with the main PS caps.

The problem could also be your source equipment, pre-amp, cdp, etc. Try this. remove your preamp from the Model 3. Short out the inputs of the Model 3. Use a shorting plug or alligator clip. Leave the speakers attached. Turn on the amp. Listen to the output from the speakers. You should have to place your ear very close to the speaker to hear anything. If after two hours the sound remains very low level, then the amp is fine and you have a problem with your upstream equipment. If it gets louder as before, then reverse your "freshening up" process and try again. If the problem still occurs, I would contact Frank for repair/upgrade assistance.

Ron

[rtate]

I'm not sure if the problem existed before my work as I only had the amp for a short time and was really only did a function check.
The hiss is there with no input present although I did not short the input connections.

[avahifi]

Unfortunately I have never seen the old VA Systems Model Three amplifier.  It was designed and produced long after I had parted company with VA Systems.  I have no service data.

One thing I did hear, the unit used a LF357 8-pin linear op-amp somewhere in the audio circuits.  I probably would be suspect of that device being a noise source in a unit of that vintage.

Regards,

Frank Van Alstine

One other thing, look at the unit on the test bench with a scope and make sure the hiss you hear is not an artifact of very high frequency oscillations.  You may have modified something into instablity with the parts changes.

[rtate]

Thanks for the respose Frank.
Is there a replacement ie. better op-amp that I could use?

[avahifi]

The National LF357 is an uncompensated linear IC (not unity gain stable).  I can't remember if it is bipolar or fet.  It likely was in a 8-pin metal "button" can configuration back in those days (over 30 years ago).  I cannot recommend a good substitution for it now, but perhaps someone else here can.

Regards,

Frank

[BradJudy]

Here's a link to the LF357 info - http://www.national.com/opf/LF/LF357.html

I'm far from an expert, but it looks like an OPA627 could be a drop-in replacement.  I'd check with someone who knows what they're talking about though.